Logo Search packages:      
Sourcecode: postgresql-8.4 version File versions

bufmgr.c

/*-------------------------------------------------------------------------
 *
 * bufmgr.c
 *      buffer manager interface routines
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *      $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.252 2009/06/11 14:49:01 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
/*
 * Principal entry points:
 *
 * ReadBuffer() -- find or create a buffer holding the requested page,
 *          and pin it so that no one can destroy it while this process
 *          is using it.
 *
 * ReleaseBuffer() -- unpin a buffer
 *
 * MarkBufferDirty() -- mark a pinned buffer's contents as "dirty".
 *          The disk write is delayed until buffer replacement or checkpoint.
 *
 * See also these files:
 *          freelist.c -- chooses victim for buffer replacement
 *          buf_table.c -- manages the buffer lookup table
 */
#include "postgres.h"

#include <sys/file.h>
#include <unistd.h>

#include "catalog/catalog.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
#include "storage/ipc.h"
#include "storage/proc.h"
#include "storage/smgr.h"
#include "utils/rel.h"
#include "utils/resowner.h"


/* Note: these two macros only work on shared buffers, not local ones! */
#define BufHdrGetBlock(bufHdr)      ((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
#define BufferGetLSN(bufHdr)  (PageGetLSN(BufHdrGetBlock(bufHdr)))

/* Note: this macro only works on local buffers, not shared ones! */
#define LocalBufHdrGetBlock(bufHdr) \
      LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]

/* Bits in SyncOneBuffer's return value */
#define BUF_WRITTEN                       0x01
#define BUF_REUSABLE                0x02


/* GUC variables */
bool        zero_damaged_pages = false;
int               bgwriter_lru_maxpages = 100;
double            bgwriter_lru_multiplier = 2.0;

/*
 * How many buffers PrefetchBuffer callers should try to stay ahead of their
 * ReadBuffer calls by.  This is maintained by the assign hook for
 * effective_io_concurrency.  Zero means "never prefetch".
 */
int               target_prefetch_pages = 0;

/* local state for StartBufferIO and related functions */
static volatile BufferDesc *InProgressBuf = NULL;
static bool IsForInput;

/* local state for LockBufferForCleanup */
static volatile BufferDesc *PinCountWaitBuf = NULL;


static Buffer ReadBuffer_common(SMgrRelation reln, bool isLocalBuf,
                          ForkNumber forkNum, BlockNumber blockNum,
                          ReadBufferMode mode, BufferAccessStrategy strategy,
                          bool *hit);
static bool PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy);
static void PinBuffer_Locked(volatile BufferDesc *buf);
static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner);
static void BufferSync(int flags);
static int  SyncOneBuffer(int buf_id, bool skip_recently_used);
static void WaitIO(volatile BufferDesc *buf);
static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
                          int set_flag_bits);
static void buffer_write_error_callback(void *arg);
static volatile BufferDesc *BufferAlloc(SMgrRelation smgr, ForkNumber forkNum,
                  BlockNumber blockNum,
                  BufferAccessStrategy strategy,
                  bool *foundPtr);
static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
static void AtProcExit_Buffers(int code, Datum arg);


/*
 * PrefetchBuffer -- initiate asynchronous read of a block of a relation
 *
 * This is named by analogy to ReadBuffer but doesn't actually allocate a
 * buffer.  Instead it tries to ensure that a future ReadBuffer for the given
 * block will not be delayed by the I/O.  Prefetching is optional.
 * No-op if prefetching isn't compiled in.
 */
void
PrefetchBuffer(Relation reln, ForkNumber forkNum, BlockNumber blockNum)
{
#ifdef USE_PREFETCH
      Assert(RelationIsValid(reln));
      Assert(BlockNumberIsValid(blockNum));

      /* Open it at the smgr level if not already done */
      RelationOpenSmgr(reln);

      if (reln->rd_istemp)
      {
            /* see comments in ReadBufferExtended */
            if (RELATION_IS_OTHER_TEMP(reln))
                  ereport(ERROR,
                              (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                        errmsg("cannot access temporary tables of other sessions")));

            /* pass it off to localbuf.c */
            LocalPrefetchBuffer(reln->rd_smgr, forkNum, blockNum);
      }
      else
      {
            BufferTag   newTag;           /* identity of requested block */
            uint32            newHash;    /* hash value for newTag */
            LWLockId    newPartitionLock; /* buffer partition lock for it */
            int               buf_id;

            /* create a tag so we can lookup the buffer */
            INIT_BUFFERTAG(newTag, reln->rd_smgr->smgr_rnode, forkNum, blockNum);

            /* determine its hash code and partition lock ID */
            newHash = BufTableHashCode(&newTag);
            newPartitionLock = BufMappingPartitionLock(newHash);

            /* see if the block is in the buffer pool already */
            LWLockAcquire(newPartitionLock, LW_SHARED);
            buf_id = BufTableLookup(&newTag, newHash);
            LWLockRelease(newPartitionLock);

            /* If not in buffers, initiate prefetch */
            if (buf_id < 0)
                  smgrprefetch(reln->rd_smgr, forkNum, blockNum);

            /*
             * If the block *is* in buffers, we do nothing.  This is not really
             * ideal: the block might be just about to be evicted, which would be
             * stupid since we know we are going to need it soon.  But the only
             * easy answer is to bump the usage_count, which does not seem like a
             * great solution: when the caller does ultimately touch the block,
             * usage_count would get bumped again, resulting in too much
             * favoritism for blocks that are involved in a prefetch sequence. A
             * real fix would involve some additional per-buffer state, and it's
             * not clear that there's enough of a problem to justify that.
             */
      }
#endif   /* USE_PREFETCH */
}


/*
 * ReadBuffer -- a shorthand for ReadBufferExtended, for reading from main
 *          fork with RBM_NORMAL mode and default strategy.
 */
Buffer
ReadBuffer(Relation reln, BlockNumber blockNum)
{
      return ReadBufferExtended(reln, MAIN_FORKNUM, blockNum, RBM_NORMAL, NULL);
}

/*
 * ReadBufferExtended -- returns a buffer containing the requested
 *          block of the requested relation.  If the blknum
 *          requested is P_NEW, extend the relation file and
 *          allocate a new block.  (Caller is responsible for
 *          ensuring that only one backend tries to extend a
 *          relation at the same time!)
 *
 * Returns: the buffer number for the buffer containing
 *          the block read.  The returned buffer has been pinned.
 *          Does not return on error --- elog's instead.
 *
 * Assume when this function is called, that reln has been opened already.
 *
 * In RBM_NORMAL mode, the page is read from disk, and the page header is
 * validated. An error is thrown if the page header is not valid.
 *
 * RBM_ZERO_ON_ERROR is like the normal mode, but if the page header is not
 * valid, the page is zeroed instead of throwing an error. This is intended
 * for non-critical data, where the caller is prepared to repair errors.
 *
 * In RBM_ZERO mode, if the page isn't in buffer cache already, it's filled
 * with zeros instead of reading it from disk.  Useful when the caller is
 * going to fill the page from scratch, since this saves I/O and avoids
 * unnecessary failure if the page-on-disk has corrupt page headers.
 * Caution: do not use this mode to read a page that is beyond the relation's
 * current physical EOF; that is likely to cause problems in md.c when
 * the page is modified and written out. P_NEW is OK, though.
 *
 * If strategy is not NULL, a nondefault buffer access strategy is used.
 * See buffer/README for details.
 */
Buffer
ReadBufferExtended(Relation reln, ForkNumber forkNum, BlockNumber blockNum,
                           ReadBufferMode mode, BufferAccessStrategy strategy)
{
      bool        hit;
      Buffer            buf;

      /* Open it at the smgr level if not already done */
      RelationOpenSmgr(reln);

      /*
       * Reject attempts to read non-local temporary relations; we would be
       * likely to get wrong data since we have no visibility into the owning
       * session's local buffers.
       */
      if (RELATION_IS_OTHER_TEMP(reln))
            ereport(ERROR,
                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                         errmsg("cannot access temporary tables of other sessions")));

      /*
       * Read the buffer, and update pgstat counters to reflect a cache hit or
       * miss.
       */
      pgstat_count_buffer_read(reln);
      buf = ReadBuffer_common(reln->rd_smgr, reln->rd_istemp, forkNum, blockNum,
                                          mode, strategy, &hit);
      if (hit)
            pgstat_count_buffer_hit(reln);
      return buf;
}


/*
 * ReadBufferWithoutRelcache -- like ReadBufferExtended, but doesn't require
 *          a relcache entry for the relation.
 *
 * NB: caller is assumed to know what it's doing if isTemp is true.
 */
Buffer
ReadBufferWithoutRelcache(RelFileNode rnode, bool isTemp,
                                      ForkNumber forkNum, BlockNumber blockNum,
                                      ReadBufferMode mode, BufferAccessStrategy strategy)
{
      bool        hit;

      SMgrRelation smgr = smgropen(rnode);

      return ReadBuffer_common(smgr, isTemp, forkNum, blockNum, mode, strategy,
                                           &hit);
}


/*
 * ReadBuffer_common -- common logic for all ReadBuffer variants
 *
 * *hit is set to true if the request was satisfied from shared buffer cache.
 */
static Buffer
ReadBuffer_common(SMgrRelation smgr, bool isLocalBuf, ForkNumber forkNum,
                          BlockNumber blockNum, ReadBufferMode mode,
                          BufferAccessStrategy strategy, bool *hit)
{
      volatile BufferDesc *bufHdr;
      Block       bufBlock;
      bool        found;
      bool        isExtend;

      *hit = false;

      /* Make sure we will have room to remember the buffer pin */
      ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

      isExtend = (blockNum == P_NEW);

      TRACE_POSTGRESQL_BUFFER_READ_START(forkNum, blockNum,
                                                         smgr->smgr_rnode.spcNode,
                                                         smgr->smgr_rnode.dbNode,
                                                         smgr->smgr_rnode.relNode,
                                                         isLocalBuf,
                                                         isExtend);

      /* Substitute proper block number if caller asked for P_NEW */
      if (isExtend)
            blockNum = smgrnblocks(smgr, forkNum);

      if (isLocalBuf)
      {
            ReadLocalBufferCount++;
            bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
            if (found)
                  LocalBufferHitCount++;
      }
      else
      {
            ReadBufferCount++;

            /*
             * lookup the buffer.  IO_IN_PROGRESS is set if the requested block is
             * not currently in memory.
             */
            bufHdr = BufferAlloc(smgr, forkNum, blockNum, strategy, &found);
            if (found)
                  BufferHitCount++;
      }

      /* At this point we do NOT hold any locks. */

      /* if it was already in the buffer pool, we're done */
      if (found)
      {
            if (!isExtend)
            {
                  /* Just need to update stats before we exit */
                  *hit = true;

                  if (VacuumCostActive)
                        VacuumCostBalance += VacuumCostPageHit;

                  TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
                                                                    smgr->smgr_rnode.spcNode,
                                                                    smgr->smgr_rnode.dbNode,
                                                                    smgr->smgr_rnode.relNode,
                                                                    isLocalBuf,
                                                                    isExtend,
                                                                    found);

                  return BufferDescriptorGetBuffer(bufHdr);
            }

            /*
             * We get here only in the corner case where we are trying to extend
             * the relation but we found a pre-existing buffer marked BM_VALID.
             * This can happen because mdread doesn't complain about reads beyond
             * EOF (when zero_damaged_pages is ON) and so a previous attempt to
             * read a block beyond EOF could have left a "valid" zero-filled
             * buffer.  Unfortunately, we have also seen this case occurring
             * because of buggy Linux kernels that sometimes return an
             * lseek(SEEK_END) result that doesn't account for a recent write. In
             * that situation, the pre-existing buffer would contain valid data
             * that we don't want to overwrite.  Since the legitimate case should
             * always have left a zero-filled buffer, complain if not PageIsNew.
             */
            bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
            if (!PageIsNew((Page) bufBlock))
                  ereport(ERROR,
                   (errmsg("unexpected data beyond EOF in block %u of relation %s",
                               blockNum, relpath(smgr->smgr_rnode, forkNum)),
                    errhint("This has been seen to occur with buggy kernels; consider updating your system.")));

            /*
             * We *must* do smgrextend before succeeding, else the page will not
             * be reserved by the kernel, and the next P_NEW call will decide to
             * return the same page.  Clear the BM_VALID bit, do the StartBufferIO
             * call that BufferAlloc didn't, and proceed.
             */
            if (isLocalBuf)
            {
                  /* Only need to adjust flags */
                  Assert(bufHdr->flags & BM_VALID);
                  bufHdr->flags &= ~BM_VALID;
            }
            else
            {
                  /*
                   * Loop to handle the very small possibility that someone re-sets
                   * BM_VALID between our clearing it and StartBufferIO inspecting
                   * it.
                   */
                  do
                  {
                        LockBufHdr(bufHdr);
                        Assert(bufHdr->flags & BM_VALID);
                        bufHdr->flags &= ~BM_VALID;
                        UnlockBufHdr(bufHdr);
                  } while (!StartBufferIO(bufHdr, true));
            }
      }

      /*
       * if we have gotten to this point, we have allocated a buffer for the
       * page but its contents are not yet valid.  IO_IN_PROGRESS is set for it,
       * if it's a shared buffer.
       *
       * Note: if smgrextend fails, we will end up with a buffer that is
       * allocated but not marked BM_VALID.  P_NEW will still select the same
       * block number (because the relation didn't get any longer on disk) and
       * so future attempts to extend the relation will find the same buffer (if
       * it's not been recycled) but come right back here to try smgrextend
       * again.
       */
      Assert(!(bufHdr->flags & BM_VALID));            /* spinlock not needed */

      bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);

      if (isExtend)
      {
            /* new buffers are zero-filled */
            MemSet((char *) bufBlock, 0, BLCKSZ);
            smgrextend(smgr, forkNum, blockNum, (char *) bufBlock, isLocalBuf);
      }
      else
      {
            /*
             * Read in the page, unless the caller intends to overwrite it and
             * just wants us to allocate a buffer.
             */
            if (mode == RBM_ZERO)
                  MemSet((char *) bufBlock, 0, BLCKSZ);
            else
            {
                  smgrread(smgr, forkNum, blockNum, (char *) bufBlock);

                  /* check for garbage data */
                  if (!PageHeaderIsValid((PageHeader) bufBlock))
                  {
                        if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
                        {
                              ereport(WARNING,
                                          (errcode(ERRCODE_DATA_CORRUPTED),
                                           errmsg("invalid page header in block %u of relation %s; zeroing out page",
                                                      blockNum,
                                                      relpath(smgr->smgr_rnode, forkNum))));
                              MemSet((char *) bufBlock, 0, BLCKSZ);
                        }
                        else
                              ereport(ERROR,
                                          (errcode(ERRCODE_DATA_CORRUPTED),
                               errmsg("invalid page header in block %u of relation %s",
                                          blockNum,
                                          relpath(smgr->smgr_rnode, forkNum))));
                  }
            }
      }

      if (isLocalBuf)
      {
            /* Only need to adjust flags */
            bufHdr->flags |= BM_VALID;
      }
      else
      {
            /* Set BM_VALID, terminate IO, and wake up any waiters */
            TerminateBufferIO(bufHdr, false, BM_VALID);
      }

      if (VacuumCostActive)
            VacuumCostBalance += VacuumCostPageMiss;

      TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
                                                        smgr->smgr_rnode.spcNode,
                                                        smgr->smgr_rnode.dbNode,
                                                        smgr->smgr_rnode.relNode,
                                                        isLocalBuf,
                                                        isExtend,
                                                        found);

      return BufferDescriptorGetBuffer(bufHdr);
}

/*
 * BufferAlloc -- subroutine for ReadBuffer.  Handles lookup of a shared
 *          buffer.  If no buffer exists already, selects a replacement
 *          victim and evicts the old page, but does NOT read in new page.
 *
 * "strategy" can be a buffer replacement strategy object, or NULL for
 * the default strategy.  The selected buffer's usage_count is advanced when
 * using the default strategy, but otherwise possibly not (see PinBuffer).
 *
 * The returned buffer is pinned and is already marked as holding the
 * desired page.  If it already did have the desired page, *foundPtr is
 * set TRUE.  Otherwise, *foundPtr is set FALSE and the buffer is marked
 * as IO_IN_PROGRESS; ReadBuffer will now need to do I/O to fill it.
 *
 * *foundPtr is actually redundant with the buffer's BM_VALID flag, but
 * we keep it for simplicity in ReadBuffer.
 *
 * No locks are held either at entry or exit.
 */
static volatile BufferDesc *
BufferAlloc(SMgrRelation smgr, ForkNumber forkNum,
                  BlockNumber blockNum,
                  BufferAccessStrategy strategy,
                  bool *foundPtr)
{
      BufferTag   newTag;                 /* identity of requested block */
      uint32            newHash;          /* hash value for newTag */
      LWLockId    newPartitionLock;       /* buffer partition lock for it */
      BufferTag   oldTag;                 /* previous identity of selected buffer */
      uint32            oldHash;          /* hash value for oldTag */
      LWLockId    oldPartitionLock;       /* buffer partition lock for it */
      BufFlags    oldFlags;
      int               buf_id;
      volatile BufferDesc *buf;
      bool        valid;

      /* create a tag so we can lookup the buffer */
      INIT_BUFFERTAG(newTag, smgr->smgr_rnode, forkNum, blockNum);

      /* determine its hash code and partition lock ID */
      newHash = BufTableHashCode(&newTag);
      newPartitionLock = BufMappingPartitionLock(newHash);

      /* see if the block is in the buffer pool already */
      LWLockAcquire(newPartitionLock, LW_SHARED);
      buf_id = BufTableLookup(&newTag, newHash);
      if (buf_id >= 0)
      {
            /*
             * Found it.  Now, pin the buffer so no one can steal it from the
             * buffer pool, and check to see if the correct data has been loaded
             * into the buffer.
             */
            buf = &BufferDescriptors[buf_id];

            valid = PinBuffer(buf, strategy);

            /* Can release the mapping lock as soon as we've pinned it */
            LWLockRelease(newPartitionLock);

            *foundPtr = TRUE;

            if (!valid)
            {
                  /*
                   * We can only get here if (a) someone else is still reading in
                   * the page, or (b) a previous read attempt failed.  We have to
                   * wait for any active read attempt to finish, and then set up our
                   * own read attempt if the page is still not BM_VALID.
                   * StartBufferIO does it all.
                   */
                  if (StartBufferIO(buf, true))
                  {
                        /*
                         * If we get here, previous attempts to read the buffer must
                         * have failed ... but we shall bravely try again.
                         */
                        *foundPtr = FALSE;
                  }
            }

            return buf;
      }

      /*
       * Didn't find it in the buffer pool.  We'll have to initialize a new
       * buffer.  Remember to unlock the mapping lock while doing the work.
       */
      LWLockRelease(newPartitionLock);

      /* Loop here in case we have to try another victim buffer */
      for (;;)
      {
            bool        lock_held;

            /*
             * Select a victim buffer.    The buffer is returned with its header
             * spinlock still held!  Also (in most cases) the BufFreelistLock is
             * still held, since it would be bad to hold the spinlock while
             * possibly waking up other processes.
             */
            buf = StrategyGetBuffer(strategy, &lock_held);

            Assert(buf->refcount == 0);

            /* Must copy buffer flags while we still hold the spinlock */
            oldFlags = buf->flags;

            /* Pin the buffer and then release the buffer spinlock */
            PinBuffer_Locked(buf);

            /* Now it's safe to release the freelist lock */
            if (lock_held)
                  LWLockRelease(BufFreelistLock);

            /*
             * If the buffer was dirty, try to write it out.  There is a race
             * condition here, in that someone might dirty it after we released it
             * above, or even while we are writing it out (since our share-lock
             * won't prevent hint-bit updates).  We will recheck the dirty bit
             * after re-locking the buffer header.
             */
            if (oldFlags & BM_DIRTY)
            {
                  /*
                   * We need a share-lock on the buffer contents to write it out
                   * (else we might write invalid data, eg because someone else is
                   * compacting the page contents while we write).  We must use a
                   * conditional lock acquisition here to avoid deadlock.  Even
                   * though the buffer was not pinned (and therefore surely not
                   * locked) when StrategyGetBuffer returned it, someone else could
                   * have pinned and exclusive-locked it by the time we get here. If
                   * we try to get the lock unconditionally, we'd block waiting for
                   * them; if they later block waiting for us, deadlock ensues.
                   * (This has been observed to happen when two backends are both
                   * trying to split btree index pages, and the second one just
                   * happens to be trying to split the page the first one got from
                   * StrategyGetBuffer.)
                   */
                  if (LWLockConditionalAcquire(buf->content_lock, LW_SHARED))
                  {
                        /*
                         * If using a nondefault strategy, and writing the buffer
                         * would require a WAL flush, let the strategy decide whether
                         * to go ahead and write/reuse the buffer or to choose another
                         * victim.  We need lock to inspect the page LSN, so this
                         * can't be done inside StrategyGetBuffer.
                         */
                        if (strategy != NULL &&
                              XLogNeedsFlush(BufferGetLSN(buf)) &&
                              StrategyRejectBuffer(strategy, buf))
                        {
                              /* Drop lock/pin and loop around for another buffer */
                              LWLockRelease(buf->content_lock);
                              UnpinBuffer(buf, true);
                              continue;
                        }

                        /* OK, do the I/O */
                        TRACE_POSTGRESQL_BUFFER_WRITE_DIRTY_START(forkNum, blockNum,
                                                                              smgr->smgr_rnode.spcNode,
                                                                               smgr->smgr_rnode.dbNode,
                                                                           smgr->smgr_rnode.relNode);

                        FlushBuffer(buf, NULL);
                        LWLockRelease(buf->content_lock);

                        TRACE_POSTGRESQL_BUFFER_WRITE_DIRTY_DONE(forkNum, blockNum,
                                                                              smgr->smgr_rnode.spcNode,
                                                                               smgr->smgr_rnode.dbNode,
                                                                           smgr->smgr_rnode.relNode);
                  }
                  else
                  {
                        /*
                         * Someone else has locked the buffer, so give it up and loop
                         * back to get another one.
                         */
                        UnpinBuffer(buf, true);
                        continue;
                  }
            }

            /*
             * To change the association of a valid buffer, we'll need to have
             * exclusive lock on both the old and new mapping partitions.
             */
            if (oldFlags & BM_TAG_VALID)
            {
                  /*
                   * Need to compute the old tag's hashcode and partition lock ID.
                   * XXX is it worth storing the hashcode in BufferDesc so we need
                   * not recompute it here?  Probably not.
                   */
                  oldTag = buf->tag;
                  oldHash = BufTableHashCode(&oldTag);
                  oldPartitionLock = BufMappingPartitionLock(oldHash);

                  /*
                   * Must lock the lower-numbered partition first to avoid
                   * deadlocks.
                   */
                  if (oldPartitionLock < newPartitionLock)
                  {
                        LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
                        LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                  }
                  else if (oldPartitionLock > newPartitionLock)
                  {
                        LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                        LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);
                  }
                  else
                  {
                        /* only one partition, only one lock */
                        LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                  }
            }
            else
            {
                  /* if it wasn't valid, we need only the new partition */
                  LWLockAcquire(newPartitionLock, LW_EXCLUSIVE);
                  /* these just keep the compiler quiet about uninit variables */
                  oldHash = 0;
                  oldPartitionLock = 0;
            }

            /*
             * Try to make a hashtable entry for the buffer under its new tag.
             * This could fail because while we were writing someone else
             * allocated another buffer for the same block we want to read in.
             * Note that we have not yet removed the hashtable entry for the old
             * tag.
             */
            buf_id = BufTableInsert(&newTag, newHash, buf->buf_id);

            if (buf_id >= 0)
            {
                  /*
                   * Got a collision. Someone has already done what we were about to
                   * do. We'll just handle this as if it were found in the buffer
                   * pool in the first place.  First, give up the buffer we were
                   * planning to use.
                   */
                  UnpinBuffer(buf, true);

                  /* Can give up that buffer's mapping partition lock now */
                  if ((oldFlags & BM_TAG_VALID) &&
                        oldPartitionLock != newPartitionLock)
                        LWLockRelease(oldPartitionLock);

                  /* remaining code should match code at top of routine */

                  buf = &BufferDescriptors[buf_id];

                  valid = PinBuffer(buf, strategy);

                  /* Can release the mapping lock as soon as we've pinned it */
                  LWLockRelease(newPartitionLock);

                  *foundPtr = TRUE;

                  if (!valid)
                  {
                        /*
                         * We can only get here if (a) someone else is still reading
                         * in the page, or (b) a previous read attempt failed.      We
                         * have to wait for any active read attempt to finish, and
                         * then set up our own read attempt if the page is still not
                         * BM_VALID.  StartBufferIO does it all.
                         */
                        if (StartBufferIO(buf, true))
                        {
                              /*
                               * If we get here, previous attempts to read the buffer
                               * must have failed ... but we shall bravely try again.
                               */
                              *foundPtr = FALSE;
                        }
                  }

                  return buf;
            }

            /*
             * Need to lock the buffer header too in order to change its tag.
             */
            LockBufHdr(buf);

            /*
             * Somebody could have pinned or re-dirtied the buffer while we were
             * doing the I/O and making the new hashtable entry.  If so, we can't
             * recycle this buffer; we must undo everything we've done and start
             * over with a new victim buffer.
             */
            oldFlags = buf->flags;
            if (buf->refcount == 1 && !(oldFlags & BM_DIRTY))
                  break;

            UnlockBufHdr(buf);
            BufTableDelete(&newTag, newHash);
            if ((oldFlags & BM_TAG_VALID) &&
                  oldPartitionLock != newPartitionLock)
                  LWLockRelease(oldPartitionLock);
            LWLockRelease(newPartitionLock);
            UnpinBuffer(buf, true);
      }

      /*
       * Okay, it's finally safe to rename the buffer.
       *
       * Clearing BM_VALID here is necessary, clearing the dirtybits is just
       * paranoia.  We also reset the usage_count since any recency of use of
       * the old content is no longer relevant.  (The usage_count starts out at
       * 1 so that the buffer can survive one clock-sweep pass.)
       */
      buf->tag = newTag;
      buf->flags &= ~(BM_VALID | BM_DIRTY | BM_JUST_DIRTIED | BM_CHECKPOINT_NEEDED | BM_IO_ERROR);
      buf->flags |= BM_TAG_VALID;
      buf->usage_count = 1;

      UnlockBufHdr(buf);

      if (oldFlags & BM_TAG_VALID)
      {
            BufTableDelete(&oldTag, oldHash);
            if (oldPartitionLock != newPartitionLock)
                  LWLockRelease(oldPartitionLock);
      }

      LWLockRelease(newPartitionLock);

      /*
       * Buffer contents are currently invalid.  Try to get the io_in_progress
       * lock.  If StartBufferIO returns false, then someone else managed to
       * read it before we did, so there's nothing left for BufferAlloc() to do.
       */
      if (StartBufferIO(buf, true))
            *foundPtr = FALSE;
      else
            *foundPtr = TRUE;

      return buf;
}

/*
 * InvalidateBuffer -- mark a shared buffer invalid and return it to the
 * freelist.
 *
 * The buffer header spinlock must be held at entry.  We drop it before
 * returning.  (This is sane because the caller must have locked the
 * buffer in order to be sure it should be dropped.)
 *
 * This is used only in contexts such as dropping a relation.  We assume
 * that no other backend could possibly be interested in using the page,
 * so the only reason the buffer might be pinned is if someone else is
 * trying to write it out.    We have to let them finish before we can
 * reclaim the buffer.
 *
 * The buffer could get reclaimed by someone else while we are waiting
 * to acquire the necessary locks; if so, don't mess it up.
 */
static void
InvalidateBuffer(volatile BufferDesc *buf)
{
      BufferTag   oldTag;
      uint32            oldHash;          /* hash value for oldTag */
      LWLockId    oldPartitionLock;       /* buffer partition lock for it */
      BufFlags    oldFlags;

      /* Save the original buffer tag before dropping the spinlock */
      oldTag = buf->tag;

      UnlockBufHdr(buf);

      /*
       * Need to compute the old tag's hashcode and partition lock ID. XXX is it
       * worth storing the hashcode in BufferDesc so we need not recompute it
       * here?  Probably not.
       */
      oldHash = BufTableHashCode(&oldTag);
      oldPartitionLock = BufMappingPartitionLock(oldHash);

retry:

      /*
       * Acquire exclusive mapping lock in preparation for changing the buffer's
       * association.
       */
      LWLockAcquire(oldPartitionLock, LW_EXCLUSIVE);

      /* Re-lock the buffer header */
      LockBufHdr(buf);

      /* If it's changed while we were waiting for lock, do nothing */
      if (!BUFFERTAGS_EQUAL(buf->tag, oldTag))
      {
            UnlockBufHdr(buf);
            LWLockRelease(oldPartitionLock);
            return;
      }

      /*
       * We assume the only reason for it to be pinned is that someone else is
       * flushing the page out.  Wait for them to finish.  (This could be an
       * infinite loop if the refcount is messed up... it would be nice to time
       * out after awhile, but there seems no way to be sure how many loops may
       * be needed.  Note that if the other guy has pinned the buffer but not
       * yet done StartBufferIO, WaitIO will fall through and we'll effectively
       * be busy-looping here.)
       */
      if (buf->refcount != 0)
      {
            UnlockBufHdr(buf);
            LWLockRelease(oldPartitionLock);
            /* safety check: should definitely not be our *own* pin */
            if (PrivateRefCount[buf->buf_id] != 0)
                  elog(ERROR, "buffer is pinned in InvalidateBuffer");
            WaitIO(buf);
            goto retry;
      }

      /*
       * Clear out the buffer's tag and flags.  We must do this to ensure that
       * linear scans of the buffer array don't think the buffer is valid.
       */
      oldFlags = buf->flags;
      CLEAR_BUFFERTAG(buf->tag);
      buf->flags = 0;
      buf->usage_count = 0;

      UnlockBufHdr(buf);

      /*
       * Remove the buffer from the lookup hashtable, if it was in there.
       */
      if (oldFlags & BM_TAG_VALID)
            BufTableDelete(&oldTag, oldHash);

      /*
       * Done with mapping lock.
       */
      LWLockRelease(oldPartitionLock);

      /*
       * Insert the buffer at the head of the list of free buffers.
       */
      StrategyFreeBuffer(buf);
}

/*
 * MarkBufferDirty
 *
 *          Marks buffer contents as dirty (actual write happens later).
 *
 * Buffer must be pinned and exclusive-locked.  (If caller does not hold
 * exclusive lock, then somebody could be in process of writing the buffer,
 * leading to risk of bad data written to disk.)
 */
void
MarkBufferDirty(Buffer buffer)
{
      volatile BufferDesc *bufHdr;

      if (!BufferIsValid(buffer))
            elog(ERROR, "bad buffer id: %d", buffer);

      if (BufferIsLocal(buffer))
      {
            MarkLocalBufferDirty(buffer);
            return;
      }

      bufHdr = &BufferDescriptors[buffer - 1];

      Assert(PrivateRefCount[buffer - 1] > 0);
      /* unfortunately we can't check if the lock is held exclusively */
      Assert(LWLockHeldByMe(bufHdr->content_lock));

      LockBufHdr(bufHdr);

      Assert(bufHdr->refcount > 0);

      /*
       * If the buffer was not dirty already, do vacuum cost accounting.
       */
      if (!(bufHdr->flags & BM_DIRTY) && VacuumCostActive)
            VacuumCostBalance += VacuumCostPageDirty;

      bufHdr->flags |= (BM_DIRTY | BM_JUST_DIRTIED);

      UnlockBufHdr(bufHdr);
}

/*
 * ReleaseAndReadBuffer -- combine ReleaseBuffer() and ReadBuffer()
 *
 * Formerly, this saved one cycle of acquiring/releasing the BufMgrLock
 * compared to calling the two routines separately.  Now it's mainly just
 * a convenience function.    However, if the passed buffer is valid and
 * already contains the desired block, we just return it as-is; and that
 * does save considerable work compared to a full release and reacquire.
 *
 * Note: it is OK to pass buffer == InvalidBuffer, indicating that no old
 * buffer actually needs to be released.  This case is the same as ReadBuffer,
 * but can save some tests in the caller.
 */
Buffer
ReleaseAndReadBuffer(Buffer buffer,
                               Relation relation,
                               BlockNumber blockNum)
{
      ForkNumber  forkNum = MAIN_FORKNUM;
      volatile BufferDesc *bufHdr;

      if (BufferIsValid(buffer))
      {
            if (BufferIsLocal(buffer))
            {
                  Assert(LocalRefCount[-buffer - 1] > 0);
                  bufHdr = &LocalBufferDescriptors[-buffer - 1];
                  if (bufHdr->tag.blockNum == blockNum &&
                        RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
                        bufHdr->tag.forkNum == forkNum)
                        return buffer;
                  ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);
                  LocalRefCount[-buffer - 1]--;
            }
            else
            {
                  Assert(PrivateRefCount[buffer - 1] > 0);
                  bufHdr = &BufferDescriptors[buffer - 1];
                  /* we have pin, so it's ok to examine tag without spinlock */
                  if (bufHdr->tag.blockNum == blockNum &&
                        RelFileNodeEquals(bufHdr->tag.rnode, relation->rd_node) &&
                        bufHdr->tag.forkNum == forkNum)
                        return buffer;
                  UnpinBuffer(bufHdr, true);
            }
      }

      return ReadBuffer(relation, blockNum);
}

/*
 * PinBuffer -- make buffer unavailable for replacement.
 *
 * For the default access strategy, the buffer's usage_count is incremented
 * when we first pin it; for other strategies we just make sure the usage_count
 * isn't zero.  (The idea of the latter is that we don't want synchronized
 * heap scans to inflate the count, but we need it to not be zero to discourage
 * other backends from stealing buffers from our ring.      As long as we cycle
 * through the ring faster than the global clock-sweep cycles, buffers in
 * our ring won't be chosen as victims for replacement by other backends.)
 *
 * This should be applied only to shared buffers, never local ones.
 *
 * Note that ResourceOwnerEnlargeBuffers must have been done already.
 *
 * Returns TRUE if buffer is BM_VALID, else FALSE.    This provision allows
 * some callers to avoid an extra spinlock cycle.
 */
static bool
PinBuffer(volatile BufferDesc *buf, BufferAccessStrategy strategy)
{
      int               b = buf->buf_id;
      bool        result;

      if (PrivateRefCount[b] == 0)
      {
            LockBufHdr(buf);
            buf->refcount++;
            if (strategy == NULL)
            {
                  if (buf->usage_count < BM_MAX_USAGE_COUNT)
                        buf->usage_count++;
            }
            else
            {
                  if (buf->usage_count == 0)
                        buf->usage_count = 1;
            }
            result = (buf->flags & BM_VALID) != 0;
            UnlockBufHdr(buf);
      }
      else
      {
            /* If we previously pinned the buffer, it must surely be valid */
            result = true;
      }
      PrivateRefCount[b]++;
      Assert(PrivateRefCount[b] > 0);
      ResourceOwnerRememberBuffer(CurrentResourceOwner,
                                                BufferDescriptorGetBuffer(buf));
      return result;
}

/*
 * PinBuffer_Locked -- as above, but caller already locked the buffer header.
 * The spinlock is released before return.
 *
 * Currently, no callers of this function want to modify the buffer's
 * usage_count at all, so there's no need for a strategy parameter.
 * Also we don't bother with a BM_VALID test (the caller could check that for
 * itself).
 *
 * Note: use of this routine is frequently mandatory, not just an optimization
 * to save a spin lock/unlock cycle, because we need to pin a buffer before
 * its state can change under us.
 */
static void
PinBuffer_Locked(volatile BufferDesc *buf)
{
      int               b = buf->buf_id;

      if (PrivateRefCount[b] == 0)
            buf->refcount++;
      UnlockBufHdr(buf);
      PrivateRefCount[b]++;
      Assert(PrivateRefCount[b] > 0);
      ResourceOwnerRememberBuffer(CurrentResourceOwner,
                                                BufferDescriptorGetBuffer(buf));
}

/*
 * UnpinBuffer -- make buffer available for replacement.
 *
 * This should be applied only to shared buffers, never local ones.
 *
 * Most but not all callers want CurrentResourceOwner to be adjusted.
 * Those that don't should pass fixOwner = FALSE.
 */
static void
UnpinBuffer(volatile BufferDesc *buf, bool fixOwner)
{
      int               b = buf->buf_id;

      if (fixOwner)
            ResourceOwnerForgetBuffer(CurrentResourceOwner,
                                                  BufferDescriptorGetBuffer(buf));

      Assert(PrivateRefCount[b] > 0);
      PrivateRefCount[b]--;
      if (PrivateRefCount[b] == 0)
      {
            /* I'd better not still hold any locks on the buffer */
            Assert(!LWLockHeldByMe(buf->content_lock));
            Assert(!LWLockHeldByMe(buf->io_in_progress_lock));

            LockBufHdr(buf);

            /* Decrement the shared reference count */
            Assert(buf->refcount > 0);
            buf->refcount--;

            /* Support LockBufferForCleanup() */
            if ((buf->flags & BM_PIN_COUNT_WAITER) &&
                  buf->refcount == 1)
            {
                  /* we just released the last pin other than the waiter's */
                  int               wait_backend_pid = buf->wait_backend_pid;

                  buf->flags &= ~BM_PIN_COUNT_WAITER;
                  UnlockBufHdr(buf);
                  ProcSendSignal(wait_backend_pid);
            }
            else
                  UnlockBufHdr(buf);
      }
}

/*
 * BufferSync -- Write out all dirty buffers in the pool.
 *
 * This is called at checkpoint time to write out all dirty shared buffers.
 * The checkpoint request flags should be passed in; currently the only one
 * examined is CHECKPOINT_IMMEDIATE, which disables delays between writes.
 */
static void
BufferSync(int flags)
{
      int               buf_id;
      int               num_to_scan;
      int               num_to_write;
      int               num_written;

      /* Make sure we can handle the pin inside SyncOneBuffer */
      ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

      /*
       * Loop over all buffers, and mark the ones that need to be written with
       * BM_CHECKPOINT_NEEDED.  Count them as we go (num_to_write), so that we
       * can estimate how much work needs to be done.
       *
       * This allows us to write only those pages that were dirty when the
       * checkpoint began, and not those that get dirtied while it proceeds.
       * Whenever a page with BM_CHECKPOINT_NEEDED is written out, either by us
       * later in this function, or by normal backends or the bgwriter cleaning
       * scan, the flag is cleared.  Any buffer dirtied after this point won't
       * have the flag set.
       *
       * Note that if we fail to write some buffer, we may leave buffers with
       * BM_CHECKPOINT_NEEDED still set.  This is OK since any such buffer would
       * certainly need to be written for the next checkpoint attempt, too.
       */
      num_to_write = 0;
      for (buf_id = 0; buf_id < NBuffers; buf_id++)
      {
            volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];

            /*
             * Header spinlock is enough to examine BM_DIRTY, see comment in
             * SyncOneBuffer.
             */
            LockBufHdr(bufHdr);

            if (bufHdr->flags & BM_DIRTY)
            {
                  bufHdr->flags |= BM_CHECKPOINT_NEEDED;
                  num_to_write++;
            }

            UnlockBufHdr(bufHdr);
      }

      if (num_to_write == 0)
            return;                             /* nothing to do */

      TRACE_POSTGRESQL_BUFFER_SYNC_START(NBuffers, num_to_write);

      /*
       * Loop over all buffers again, and write the ones (still) marked with
       * BM_CHECKPOINT_NEEDED.  In this loop, we start at the clock sweep point
       * since we might as well dump soon-to-be-recycled buffers first.
       *
       * Note that we don't read the buffer alloc count here --- that should be
       * left untouched till the next BgBufferSync() call.
       */
      buf_id = StrategySyncStart(NULL, NULL);
      num_to_scan = NBuffers;
      num_written = 0;
      while (num_to_scan-- > 0)
      {
            volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];

            /*
             * We don't need to acquire the lock here, because we're only looking
             * at a single bit. It's possible that someone else writes the buffer
             * and clears the flag right after we check, but that doesn't matter
             * since SyncOneBuffer will then do nothing.  However, there is a
             * further race condition: it's conceivable that between the time we
             * examine the bit here and the time SyncOneBuffer acquires lock,
             * someone else not only wrote the buffer but replaced it with another
             * page and dirtied it.  In that improbable case, SyncOneBuffer will
             * write the buffer though we didn't need to.  It doesn't seem worth
             * guarding against this, though.
             */
            if (bufHdr->flags & BM_CHECKPOINT_NEEDED)
            {
                  if (SyncOneBuffer(buf_id, false) & BUF_WRITTEN)
                  {
                        TRACE_POSTGRESQL_BUFFER_SYNC_WRITTEN(buf_id);
                        BgWriterStats.m_buf_written_checkpoints++;
                        num_written++;

                        /*
                         * We know there are at most num_to_write buffers with
                         * BM_CHECKPOINT_NEEDED set; so we can stop scanning if
                         * num_written reaches num_to_write.
                         *
                         * Note that num_written doesn't include buffers written by
                         * other backends, or by the bgwriter cleaning scan. That
                         * means that the estimate of how much progress we've made is
                         * conservative, and also that this test will often fail to
                         * trigger.  But it seems worth making anyway.
                         */
                        if (num_written >= num_to_write)
                              break;

                        /*
                         * Perform normal bgwriter duties and sleep to throttle our
                         * I/O rate.
                         */
                        CheckpointWriteDelay(flags,
                                                       (double) num_written / num_to_write);
                  }
            }

            if (++buf_id >= NBuffers)
                  buf_id = 0;
      }

      /*
       * Update checkpoint statistics. As noted above, this doesn't include
       * buffers written by other backends or bgwriter scan.
       */
      CheckpointStats.ckpt_bufs_written += num_written;

      TRACE_POSTGRESQL_BUFFER_SYNC_DONE(NBuffers, num_written, num_to_write);
}

/*
 * BgBufferSync -- Write out some dirty buffers in the pool.
 *
 * This is called periodically by the background writer process.
 */
void
BgBufferSync(void)
{
      /* info obtained from freelist.c */
      int               strategy_buf_id;
      uint32            strategy_passes;
      uint32            recent_alloc;

      /*
       * Information saved between calls so we can determine the strategy
       * point's advance rate and avoid scanning already-cleaned buffers.
       */
      static bool saved_info_valid = false;
      static int  prev_strategy_buf_id;
      static uint32 prev_strategy_passes;
      static int  next_to_clean;
      static uint32 next_passes;

      /* Moving averages of allocation rate and clean-buffer density */
      static float smoothed_alloc = 0;
      static float smoothed_density = 10.0;

      /* Potentially these could be tunables, but for now, not */
      float       smoothing_samples = 16;
      float       scan_whole_pool_milliseconds = 120000.0;

      /* Used to compute how far we scan ahead */
      long        strategy_delta;
      int               bufs_to_lap;
      int               bufs_ahead;
      float       scans_per_alloc;
      int               reusable_buffers_est;
      int               upcoming_alloc_est;
      int               min_scan_buffers;

      /* Variables for the scanning loop proper */
      int               num_to_scan;
      int               num_written;
      int               reusable_buffers;

      /*
       * Find out where the freelist clock sweep currently is, and how many
       * buffer allocations have happened since our last call.
       */
      strategy_buf_id = StrategySyncStart(&strategy_passes, &recent_alloc);

      /* Report buffer alloc counts to pgstat */
      BgWriterStats.m_buf_alloc += recent_alloc;

      /*
       * If we're not running the LRU scan, just stop after doing the stats
       * stuff.  We mark the saved state invalid so that we can recover sanely
       * if LRU scan is turned back on later.
       */
      if (bgwriter_lru_maxpages <= 0)
      {
            saved_info_valid = false;
            return;
      }

      /*
       * Compute strategy_delta = how many buffers have been scanned by the
       * clock sweep since last time.  If first time through, assume none. Then
       * see if we are still ahead of the clock sweep, and if so, how many
       * buffers we could scan before we'd catch up with it and "lap" it. Note:
       * weird-looking coding of xxx_passes comparisons are to avoid bogus
       * behavior when the passes counts wrap around.
       */
      if (saved_info_valid)
      {
            int32       passes_delta = strategy_passes - prev_strategy_passes;

            strategy_delta = strategy_buf_id - prev_strategy_buf_id;
            strategy_delta += (long) passes_delta *NBuffers;

            Assert(strategy_delta >= 0);

            if ((int32) (next_passes - strategy_passes) > 0)
            {
                  /* we're one pass ahead of the strategy point */
                  bufs_to_lap = strategy_buf_id - next_to_clean;
#ifdef BGW_DEBUG
                  elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
                         next_passes, next_to_clean,
                         strategy_passes, strategy_buf_id,
                         strategy_delta, bufs_to_lap);
#endif
            }
            else if (next_passes == strategy_passes &&
                         next_to_clean >= strategy_buf_id)
            {
                  /* on same pass, but ahead or at least not behind */
                  bufs_to_lap = NBuffers - (next_to_clean - strategy_buf_id);
#ifdef BGW_DEBUG
                  elog(DEBUG2, "bgwriter ahead: bgw %u-%u strategy %u-%u delta=%ld lap=%d",
                         next_passes, next_to_clean,
                         strategy_passes, strategy_buf_id,
                         strategy_delta, bufs_to_lap);
#endif
            }
            else
            {
                  /*
                   * We're behind, so skip forward to the strategy point and start
                   * cleaning from there.
                   */
#ifdef BGW_DEBUG
                  elog(DEBUG2, "bgwriter behind: bgw %u-%u strategy %u-%u delta=%ld",
                         next_passes, next_to_clean,
                         strategy_passes, strategy_buf_id,
                         strategy_delta);
#endif
                  next_to_clean = strategy_buf_id;
                  next_passes = strategy_passes;
                  bufs_to_lap = NBuffers;
            }
      }
      else
      {
            /*
             * Initializing at startup or after LRU scanning had been off. Always
             * start at the strategy point.
             */
#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter initializing: strategy %u-%u",
                   strategy_passes, strategy_buf_id);
#endif
            strategy_delta = 0;
            next_to_clean = strategy_buf_id;
            next_passes = strategy_passes;
            bufs_to_lap = NBuffers;
      }

      /* Update saved info for next time */
      prev_strategy_buf_id = strategy_buf_id;
      prev_strategy_passes = strategy_passes;
      saved_info_valid = true;

      /*
       * Compute how many buffers had to be scanned for each new allocation, ie,
       * 1/density of reusable buffers, and track a moving average of that.
       *
       * If the strategy point didn't move, we don't update the density estimate
       */
      if (strategy_delta > 0 && recent_alloc > 0)
      {
            scans_per_alloc = (float) strategy_delta / (float) recent_alloc;
            smoothed_density += (scans_per_alloc - smoothed_density) /
                  smoothing_samples;
      }

      /*
       * Estimate how many reusable buffers there are between the current
       * strategy point and where we've scanned ahead to, based on the smoothed
       * density estimate.
       */
      bufs_ahead = NBuffers - bufs_to_lap;
      reusable_buffers_est = (float) bufs_ahead / smoothed_density;

      /*
       * Track a moving average of recent buffer allocations.  Here, rather than
       * a true average we want a fast-attack, slow-decline behavior: we
       * immediately follow any increase.
       */
      if (smoothed_alloc <= (float) recent_alloc)
            smoothed_alloc = recent_alloc;
      else
            smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
                  smoothing_samples;

      /* Scale the estimate by a GUC to allow more aggressive tuning. */
      upcoming_alloc_est = smoothed_alloc * bgwriter_lru_multiplier;

      /*
       * Even in cases where there's been little or no buffer allocation
       * activity, we want to make a small amount of progress through the buffer
       * cache so that as many reusable buffers as possible are clean after an
       * idle period.
       *
       * (scan_whole_pool_milliseconds / BgWriterDelay) computes how many times
       * the BGW will be called during the scan_whole_pool time; slice the
       * buffer pool into that many sections.
       */
      min_scan_buffers = (int) (NBuffers / (scan_whole_pool_milliseconds / BgWriterDelay));

      if (upcoming_alloc_est < (min_scan_buffers + reusable_buffers_est))
      {
#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter: alloc_est=%d too small, using min=%d + reusable_est=%d",
                   upcoming_alloc_est, min_scan_buffers, reusable_buffers_est);
#endif
            upcoming_alloc_est = min_scan_buffers + reusable_buffers_est;
      }

      /*
       * Now write out dirty reusable buffers, working forward from the
       * next_to_clean point, until we have lapped the strategy scan, or cleaned
       * enough buffers to match our estimate of the next cycle's allocation
       * requirements, or hit the bgwriter_lru_maxpages limit.
       */

      /* Make sure we can handle the pin inside SyncOneBuffer */
      ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

      num_to_scan = bufs_to_lap;
      num_written = 0;
      reusable_buffers = reusable_buffers_est;

      /* Execute the LRU scan */
      while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
      {
            int               buffer_state = SyncOneBuffer(next_to_clean, true);

            if (++next_to_clean >= NBuffers)
            {
                  next_to_clean = 0;
                  next_passes++;
            }
            num_to_scan--;

            if (buffer_state & BUF_WRITTEN)
            {
                  reusable_buffers++;
                  if (++num_written >= bgwriter_lru_maxpages)
                  {
                        BgWriterStats.m_maxwritten_clean++;
                        break;
                  }
            }
            else if (buffer_state & BUF_REUSABLE)
                  reusable_buffers++;
      }

      BgWriterStats.m_buf_written_clean += num_written;

#ifdef BGW_DEBUG
      elog(DEBUG1, "bgwriter: recent_alloc=%u smoothed=%.2f delta=%ld ahead=%d density=%.2f reusable_est=%d upcoming_est=%d scanned=%d wrote=%d reusable=%d",
             recent_alloc, smoothed_alloc, strategy_delta, bufs_ahead,
             smoothed_density, reusable_buffers_est, upcoming_alloc_est,
             bufs_to_lap - num_to_scan,
             num_written,
             reusable_buffers - reusable_buffers_est);
#endif

      /*
       * Consider the above scan as being like a new allocation scan.
       * Characterize its density and update the smoothed one based on it. This
       * effectively halves the moving average period in cases where both the
       * strategy and the background writer are doing some useful scanning,
       * which is helpful because a long memory isn't as desirable on the
       * density estimates.
       */
      strategy_delta = bufs_to_lap - num_to_scan;
      recent_alloc = reusable_buffers - reusable_buffers_est;
      if (strategy_delta > 0 && recent_alloc > 0)
      {
            scans_per_alloc = (float) strategy_delta / (float) recent_alloc;
            smoothed_density += (scans_per_alloc - smoothed_density) /
                  smoothing_samples;

#ifdef BGW_DEBUG
            elog(DEBUG2, "bgwriter: cleaner density alloc=%u scan=%ld density=%.2f new smoothed=%.2f",
                   recent_alloc, strategy_delta, scans_per_alloc, smoothed_density);
#endif
      }
}

/*
 * SyncOneBuffer -- process a single buffer during syncing.
 *
 * If skip_recently_used is true, we don't write currently-pinned buffers, nor
 * buffers marked recently used, as these are not replacement candidates.
 *
 * Returns a bitmask containing the following flag bits:
 *    BUF_WRITTEN: we wrote the buffer.
 *    BUF_REUSABLE: buffer is available for replacement, ie, it has
 *          pin count 0 and usage count 0.
 *
 * (BUF_WRITTEN could be set in error if FlushBuffers finds the buffer clean
 * after locking it, but we don't care all that much.)
 *
 * Note: caller must have done ResourceOwnerEnlargeBuffers.
 */
static int
SyncOneBuffer(int buf_id, bool skip_recently_used)
{
      volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
      int               result = 0;

      /*
       * Check whether buffer needs writing.
       *
       * We can make this check without taking the buffer content lock so long
       * as we mark pages dirty in access methods *before* logging changes with
       * XLogInsert(): if someone marks the buffer dirty just after our check we
       * don't worry because our checkpoint.redo points before log record for
       * upcoming changes and so we are not required to write such dirty buffer.
       */
      LockBufHdr(bufHdr);

      if (bufHdr->refcount == 0 && bufHdr->usage_count == 0)
            result |= BUF_REUSABLE;
      else if (skip_recently_used)
      {
            /* Caller told us not to write recently-used buffers */
            UnlockBufHdr(bufHdr);
            return result;
      }

      if (!(bufHdr->flags & BM_VALID) || !(bufHdr->flags & BM_DIRTY))
      {
            /* It's clean, so nothing to do */
            UnlockBufHdr(bufHdr);
            return result;
      }

      /*
       * Pin it, share-lock it, write it.  (FlushBuffer will do nothing if the
       * buffer is clean by the time we've locked it.)
       */
      PinBuffer_Locked(bufHdr);
      LWLockAcquire(bufHdr->content_lock, LW_SHARED);

      FlushBuffer(bufHdr, NULL);

      LWLockRelease(bufHdr->content_lock);
      UnpinBuffer(bufHdr, true);

      return result | BUF_WRITTEN;
}


/*
 * Return a palloc'd string containing buffer usage statistics.
 */
char *
ShowBufferUsage(void)
{
      StringInfoData str;
      float       hitrate;
      float       localhitrate;

      initStringInfo(&str);

      if (ReadBufferCount == 0)
            hitrate = 0.0;
      else
            hitrate = (float) BufferHitCount *100.0 / ReadBufferCount;

      if (ReadLocalBufferCount == 0)
            localhitrate = 0.0;
      else
            localhitrate = (float) LocalBufferHitCount *100.0 / ReadLocalBufferCount;

      appendStringInfo(&str,
      "!\tShared blocks: %10ld read, %10ld written, buffer hit rate = %.2f%%\n",
                        ReadBufferCount - BufferHitCount, BufferFlushCount, hitrate);
      appendStringInfo(&str,
      "!\tLocal  blocks: %10ld read, %10ld written, buffer hit rate = %.2f%%\n",
                               ReadLocalBufferCount - LocalBufferHitCount, LocalBufferFlushCount, localhitrate);
      appendStringInfo(&str,
                               "!\tDirect blocks: %10ld read, %10ld written\n",
                               BufFileReadCount, BufFileWriteCount);

      return str.data;
}

void
ResetBufferUsage(void)
{
      BufferHitCount = 0;
      ReadBufferCount = 0;
      BufferFlushCount = 0;
      LocalBufferHitCount = 0;
      ReadLocalBufferCount = 0;
      LocalBufferFlushCount = 0;
      BufFileReadCount = 0;
      BufFileWriteCount = 0;
}

/*
 *          AtEOXact_Buffers - clean up at end of transaction.
 *
 *          As of PostgreSQL 8.0, buffer pins should get released by the
 *          ResourceOwner mechanism.  This routine is just a debugging
 *          cross-check that no pins remain.
 */
void
AtEOXact_Buffers(bool isCommit)
{
#ifdef USE_ASSERT_CHECKING
      if (assert_enabled)
      {
            int               i;

            for (i = 0; i < NBuffers; i++)
            {
                  Assert(PrivateRefCount[i] == 0);
            }
      }
#endif

      AtEOXact_LocalBuffers(isCommit);
}

/*
 * InitBufferPoolBackend --- second-stage initialization of a new backend
 *
 * This is called after we have acquired a PGPROC and so can safely get
 * LWLocks.  We don't currently need to do anything at this stage ...
 * except register a shmem-exit callback.  AtProcExit_Buffers needs LWLock
 * access, and thereby has to be called at the corresponding phase of
 * backend shutdown.
 */
void
InitBufferPoolBackend(void)
{
      on_shmem_exit(AtProcExit_Buffers, 0);
}

/*
 * Ensure we have released all shared-buffer locks and pins during backend exit
 */
static void
AtProcExit_Buffers(int code, Datum arg)
{
      int               i;

      AbortBufferIO();
      UnlockBuffers();

      for (i = 0; i < NBuffers; i++)
      {
            if (PrivateRefCount[i] != 0)
            {
                  volatile BufferDesc *buf = &(BufferDescriptors[i]);

                  /*
                   * We don't worry about updating ResourceOwner; if we even got
                   * here, it suggests that ResourceOwners are messed up.
                   */
                  PrivateRefCount[i] = 1;       /* make sure we release shared pin */
                  UnpinBuffer(buf, false);
                  Assert(PrivateRefCount[i] == 0);
            }
      }

      /* localbuf.c needs a chance too */
      AtProcExit_LocalBuffers();
}

/*
 * Helper routine to issue warnings when a buffer is unexpectedly pinned
 */
void
PrintBufferLeakWarning(Buffer buffer)
{
      volatile BufferDesc *buf;
      int32       loccount;
      char     *path;

      Assert(BufferIsValid(buffer));
      if (BufferIsLocal(buffer))
      {
            buf = &LocalBufferDescriptors[-buffer - 1];
            loccount = LocalRefCount[-buffer - 1];
      }
      else
      {
            buf = &BufferDescriptors[buffer - 1];
            loccount = PrivateRefCount[buffer - 1];
      }

      /* theoretically we should lock the bufhdr here */
      path = relpath(buf->tag.rnode, buf->tag.forkNum);
      elog(WARNING,
             "buffer refcount leak: [%03d] "
             "(rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
             buffer, path,
             buf->tag.blockNum, buf->flags,
             buf->refcount, loccount);
      pfree(path);
}

/*
 * CheckPointBuffers
 *
 * Flush all dirty blocks in buffer pool to disk at checkpoint time.
 *
 * Note: temporary relations do not participate in checkpoints, so they don't
 * need to be flushed.
 */
void
CheckPointBuffers(int flags)
{
      TRACE_POSTGRESQL_BUFFER_CHECKPOINT_START(flags);
      CheckpointStats.ckpt_write_t = GetCurrentTimestamp();
      BufferSync(flags);
      CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
      TRACE_POSTGRESQL_BUFFER_CHECKPOINT_SYNC_START();
      smgrsync();
      CheckpointStats.ckpt_sync_end_t = GetCurrentTimestamp();
      TRACE_POSTGRESQL_BUFFER_CHECKPOINT_DONE();
}


/*
 * Do whatever is needed to prepare for commit at the bufmgr and smgr levels
 */
void
BufmgrCommit(void)
{
      /* Nothing to do in bufmgr anymore... */
}

/*
 * BufferGetBlockNumber
 *          Returns the block number associated with a buffer.
 *
 * Note:
 *          Assumes that the buffer is valid and pinned, else the
 *          value may be obsolete immediately...
 */
BlockNumber
BufferGetBlockNumber(Buffer buffer)
{
      volatile BufferDesc *bufHdr;

      Assert(BufferIsPinned(buffer));

      if (BufferIsLocal(buffer))
            bufHdr = &(LocalBufferDescriptors[-buffer - 1]);
      else
            bufHdr = &BufferDescriptors[buffer - 1];

      /* pinned, so OK to read tag without spinlock */
      return bufHdr->tag.blockNum;
}

/*
 * BufferGetTag
 *          Returns the relfilenode, fork number and block number associated with
 *          a buffer.
 */
void
BufferGetTag(Buffer buffer, RelFileNode *rnode, ForkNumber *forknum,
                   BlockNumber *blknum)
{
      volatile BufferDesc *bufHdr;

      /* Do the same checks as BufferGetBlockNumber. */
      Assert(BufferIsPinned(buffer));

      if (BufferIsLocal(buffer))
            bufHdr = &(LocalBufferDescriptors[-buffer - 1]);
      else
            bufHdr = &BufferDescriptors[buffer - 1];

      /* pinned, so OK to read tag without spinlock */
      *rnode = bufHdr->tag.rnode;
      *forknum = bufHdr->tag.forkNum;
      *blknum = bufHdr->tag.blockNum;
}

/*
 * FlushBuffer
 *          Physically write out a shared buffer.
 *
 * NOTE: this actually just passes the buffer contents to the kernel; the
 * real write to disk won't happen until the kernel feels like it.  This
 * is okay from our point of view since we can redo the changes from WAL.
 * However, we will need to force the changes to disk via fsync before
 * we can checkpoint WAL.
 *
 * The caller must hold a pin on the buffer and have share-locked the
 * buffer contents.  (Note: a share-lock does not prevent updates of
 * hint bits in the buffer, so the page could change while the write
 * is in progress, but we assume that that will not invalidate the data
 * written.)
 *
 * If the caller has an smgr reference for the buffer's relation, pass it
 * as the second parameter.  If not, pass NULL.
 */
static void
FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln)
{
      XLogRecPtr  recptr;
      ErrorContextCallback errcontext;

      /*
       * Acquire the buffer's io_in_progress lock.  If StartBufferIO returns
       * false, then someone else flushed the buffer before we could, so we need
       * not do anything.
       */
      if (!StartBufferIO(buf, false))
            return;

      /* Setup error traceback support for ereport() */
      errcontext.callback = buffer_write_error_callback;
      errcontext.arg = (void *) buf;
      errcontext.previous = error_context_stack;
      error_context_stack = &errcontext;

      /* Find smgr relation for buffer */
      if (reln == NULL)
            reln = smgropen(buf->tag.rnode);

      TRACE_POSTGRESQL_BUFFER_FLUSH_START(buf->tag.forkNum,
                                                            buf->tag.blockNum,
                                                            reln->smgr_rnode.spcNode,
                                                            reln->smgr_rnode.dbNode,
                                                            reln->smgr_rnode.relNode);

      /*
       * Force XLOG flush up to buffer's LSN.  This implements the basic WAL
       * rule that log updates must hit disk before any of the data-file changes
       * they describe do.
       */
      recptr = BufferGetLSN(buf);
      XLogFlush(recptr);

      /*
       * Now it's safe to write buffer to disk. Note that no one else should
       * have been able to write it while we were busy with log flushing because
       * we have the io_in_progress lock.
       */

      /* To check if block content changes while flushing. - vadim 01/17/97 */
      LockBufHdr(buf);
      buf->flags &= ~BM_JUST_DIRTIED;
      UnlockBufHdr(buf);

      smgrwrite(reln,
                    buf->tag.forkNum,
                    buf->tag.blockNum,
                    (char *) BufHdrGetBlock(buf),
                    false);

      BufferFlushCount++;

      /*
       * Mark the buffer as clean (unless BM_JUST_DIRTIED has become set) and
       * end the io_in_progress state.
       */
      TerminateBufferIO(buf, true, 0);

      TRACE_POSTGRESQL_BUFFER_FLUSH_DONE(buf->tag.forkNum,
                                                         buf->tag.blockNum,
                                                         reln->smgr_rnode.spcNode,
                                                         reln->smgr_rnode.dbNode,
                                                         reln->smgr_rnode.relNode);

      /* Pop the error context stack */
      error_context_stack = errcontext.previous;
}

/*
 * RelationGetNumberOfBlocks
 *          Determines the current number of pages in the relation.
 */
BlockNumber
RelationGetNumberOfBlocks(Relation relation)
{
      /* Open it at the smgr level if not already done */
      RelationOpenSmgr(relation);

      return smgrnblocks(relation->rd_smgr, MAIN_FORKNUM);
}

/* ---------------------------------------------------------------------
 *          DropRelFileNodeBuffers
 *
 *          This function removes from the buffer pool all the pages of the
 *          specified relation that have block numbers >= firstDelBlock.
 *          (In particular, with firstDelBlock = 0, all pages are removed.)
 *          Dirty pages are simply dropped, without bothering to write them
 *          out first.  Therefore, this is NOT rollback-able, and so should be
 *          used only with extreme caution!
 *
 *          Currently, this is called only from smgr.c when the underlying file
 *          is about to be deleted or truncated (firstDelBlock is needed for
 *          the truncation case).  The data in the affected pages would therefore
 *          be deleted momentarily anyway, and there is no point in writing it.
 *          It is the responsibility of higher-level code to ensure that the
 *          deletion or truncation does not lose any data that could be needed
 *          later.      It is also the responsibility of higher-level code to ensure
 *          that no other process could be trying to load more pages of the
 *          relation into buffers.
 *
 *          XXX currently it sequentially searches the buffer pool, should be
 *          changed to more clever ways of searching.  However, this routine
 *          is used only in code paths that aren't very performance-critical,
 *          and we shouldn't slow down the hot paths to make it faster ...
 * --------------------------------------------------------------------
 */
void
DropRelFileNodeBuffers(RelFileNode rnode, ForkNumber forkNum, bool istemp,
                                 BlockNumber firstDelBlock)
{
      int               i;

      if (istemp)
      {
            DropRelFileNodeLocalBuffers(rnode, forkNum, firstDelBlock);
            return;
      }

      for (i = 0; i < NBuffers; i++)
      {
            volatile BufferDesc *bufHdr = &BufferDescriptors[i];

            LockBufHdr(bufHdr);
            if (RelFileNodeEquals(bufHdr->tag.rnode, rnode) &&
                  bufHdr->tag.forkNum == forkNum &&
                  bufHdr->tag.blockNum >= firstDelBlock)
                  InvalidateBuffer(bufHdr);     /* releases spinlock */
            else
                  UnlockBufHdr(bufHdr);
      }
}

/* ---------------------------------------------------------------------
 *          DropDatabaseBuffers
 *
 *          This function removes all the buffers in the buffer cache for a
 *          particular database.  Dirty pages are simply dropped, without
 *          bothering to write them out first.  This is used when we destroy a
 *          database, to avoid trying to flush data to disk when the directory
 *          tree no longer exists.  Implementation is pretty similar to
 *          DropRelFileNodeBuffers() which is for destroying just one relation.
 * --------------------------------------------------------------------
 */
void
DropDatabaseBuffers(Oid dbid)
{
      int               i;
      volatile BufferDesc *bufHdr;

      /*
       * We needn't consider local buffers, since by assumption the target
       * database isn't our own.
       */

      for (i = 0; i < NBuffers; i++)
      {
            bufHdr = &BufferDescriptors[i];
            LockBufHdr(bufHdr);
            if (bufHdr->tag.rnode.dbNode == dbid)
                  InvalidateBuffer(bufHdr);     /* releases spinlock */
            else
                  UnlockBufHdr(bufHdr);
      }
}

/* -----------------------------------------------------------------
 *          PrintBufferDescs
 *
 *          this function prints all the buffer descriptors, for debugging
 *          use only.
 * -----------------------------------------------------------------
 */
#ifdef NOT_USED
void
PrintBufferDescs(void)
{
      int               i;
      volatile BufferDesc *buf = BufferDescriptors;

      for (i = 0; i < NBuffers; ++i, ++buf)
      {
            /* theoretically we should lock the bufhdr here */
            elog(LOG,
                   "[%02d] (freeNext=%d, rel=%s, "
                   "blockNum=%u, flags=0x%x, refcount=%u %d)",
                   i, buf->freeNext,
                   relpath(buf->tag.rnode, buf->tag.forkNum),
                   buf->tag.blockNum, buf->flags,
                   buf->refcount, PrivateRefCount[i]);
      }
}
#endif

#ifdef NOT_USED
void
PrintPinnedBufs(void)
{
      int               i;
      volatile BufferDesc *buf = BufferDescriptors;

      for (i = 0; i < NBuffers; ++i, ++buf)
      {
            if (PrivateRefCount[i] > 0)
            {
                  /* theoretically we should lock the bufhdr here */
                  elog(LOG,
                         "[%02d] (freeNext=%d, rel=%s, "
                         "blockNum=%u, flags=0x%x, refcount=%u %d)",
                         i, buf->freeNext,
                         relpath(buf->tag.rnode, buf->tag.forkNum),
                         buf->tag.blockNum, buf->flags,
                         buf->refcount, PrivateRefCount[i]);
            }
      }
}
#endif

/* ---------------------------------------------------------------------
 *          FlushRelationBuffers
 *
 *          This function writes all dirty pages of a relation out to disk
 *          (or more accurately, out to kernel disk buffers), ensuring that the
 *          kernel has an up-to-date view of the relation.
 *
 *          Generally, the caller should be holding AccessExclusiveLock on the
 *          target relation to ensure that no other backend is busy dirtying
 *          more blocks of the relation; the effects can't be expected to last
 *          after the lock is released.
 *
 *          XXX currently it sequentially searches the buffer pool, should be
 *          changed to more clever ways of searching.  This routine is not
 *          used in any performance-critical code paths, so it's not worth
 *          adding additional overhead to normal paths to make it go faster;
 *          but see also DropRelFileNodeBuffers.
 * --------------------------------------------------------------------
 */
void
FlushRelationBuffers(Relation rel)
{
      int               i;
      volatile BufferDesc *bufHdr;

      /* Open rel at the smgr level if not already done */
      RelationOpenSmgr(rel);

      if (rel->rd_istemp)
      {
            for (i = 0; i < NLocBuffer; i++)
            {
                  bufHdr = &LocalBufferDescriptors[i];
                  if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
                        (bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_DIRTY))
                  {
                        ErrorContextCallback errcontext;

                        /* Setup error traceback support for ereport() */
                        errcontext.callback = buffer_write_error_callback;
                        errcontext.arg = (void *) bufHdr;
                        errcontext.previous = error_context_stack;
                        error_context_stack = &errcontext;

                        smgrwrite(rel->rd_smgr,
                                      bufHdr->tag.forkNum,
                                      bufHdr->tag.blockNum,
                                      (char *) LocalBufHdrGetBlock(bufHdr),
                                      true);

                        bufHdr->flags &= ~(BM_DIRTY | BM_JUST_DIRTIED);

                        /* Pop the error context stack */
                        error_context_stack = errcontext.previous;
                  }
            }

            return;
      }

      /* Make sure we can handle the pin inside the loop */
      ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

      for (i = 0; i < NBuffers; i++)
      {
            bufHdr = &BufferDescriptors[i];
            LockBufHdr(bufHdr);
            if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
                  (bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_DIRTY))
            {
                  PinBuffer_Locked(bufHdr);
                  LWLockAcquire(bufHdr->content_lock, LW_SHARED);
                  FlushBuffer(bufHdr, rel->rd_smgr);
                  LWLockRelease(bufHdr->content_lock);
                  UnpinBuffer(bufHdr, true);
            }
            else
                  UnlockBufHdr(bufHdr);
      }
}

/* ---------------------------------------------------------------------
 *          FlushDatabaseBuffers
 *
 *          This function writes all dirty pages of a database out to disk
 *          (or more accurately, out to kernel disk buffers), ensuring that the
 *          kernel has an up-to-date view of the database.
 *
 *          Generally, the caller should be holding an appropriate lock to ensure
 *          no other backend is active in the target database; otherwise more
 *          pages could get dirtied.
 *
 *          Note we don't worry about flushing any pages of temporary relations.
 *          It's assumed these wouldn't be interesting.
 * --------------------------------------------------------------------
 */
void
FlushDatabaseBuffers(Oid dbid)
{
      int               i;
      volatile BufferDesc *bufHdr;

      /* Make sure we can handle the pin inside the loop */
      ResourceOwnerEnlargeBuffers(CurrentResourceOwner);

      for (i = 0; i < NBuffers; i++)
      {
            bufHdr = &BufferDescriptors[i];
            LockBufHdr(bufHdr);
            if (bufHdr->tag.rnode.dbNode == dbid &&
                  (bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_DIRTY))
            {
                  PinBuffer_Locked(bufHdr);
                  LWLockAcquire(bufHdr->content_lock, LW_SHARED);
                  FlushBuffer(bufHdr, NULL);
                  LWLockRelease(bufHdr->content_lock);
                  UnpinBuffer(bufHdr, true);
            }
            else
                  UnlockBufHdr(bufHdr);
      }
}

/*
 * ReleaseBuffer -- release the pin on a buffer
 */
void
ReleaseBuffer(Buffer buffer)
{
      volatile BufferDesc *bufHdr;

      if (!BufferIsValid(buffer))
            elog(ERROR, "bad buffer id: %d", buffer);

      ResourceOwnerForgetBuffer(CurrentResourceOwner, buffer);

      if (BufferIsLocal(buffer))
      {
            Assert(LocalRefCount[-buffer - 1] > 0);
            LocalRefCount[-buffer - 1]--;
            return;
      }

      bufHdr = &BufferDescriptors[buffer - 1];

      Assert(PrivateRefCount[buffer - 1] > 0);

      if (PrivateRefCount[buffer - 1] > 1)
            PrivateRefCount[buffer - 1]--;
      else
            UnpinBuffer(bufHdr, false);
}

/*
 * UnlockReleaseBuffer -- release the content lock and pin on a buffer
 *
 * This is just a shorthand for a common combination.
 */
void
UnlockReleaseBuffer(Buffer buffer)
{
      LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
      ReleaseBuffer(buffer);
}

/*
 * IncrBufferRefCount
 *          Increment the pin count on a buffer that we have *already* pinned
 *          at least once.
 *
 *          This function cannot be used on a buffer we do not have pinned,
 *          because it doesn't change the shared buffer state.
 */
void
IncrBufferRefCount(Buffer buffer)
{
      Assert(BufferIsPinned(buffer));
      ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
      ResourceOwnerRememberBuffer(CurrentResourceOwner, buffer);
      if (BufferIsLocal(buffer))
            LocalRefCount[-buffer - 1]++;
      else
            PrivateRefCount[buffer - 1]++;
}

/*
 * SetBufferCommitInfoNeedsSave
 *
 *    Mark a buffer dirty when we have updated tuple commit-status bits in it.
 *
 * This is essentially the same as MarkBufferDirty, except that the caller
 * might have only share-lock instead of exclusive-lock on the buffer's
 * content lock.  We preserve the distinction mainly as a way of documenting
 * that the caller has not made a critical data change --- the status-bit
 * update could be redone by someone else just as easily.  Therefore, no WAL
 * log record need be generated, whereas calls to MarkBufferDirty really ought
 * to be associated with a WAL-entry-creating action.
 */
void
SetBufferCommitInfoNeedsSave(Buffer buffer)
{
      volatile BufferDesc *bufHdr;

      if (!BufferIsValid(buffer))
            elog(ERROR, "bad buffer id: %d", buffer);

      if (BufferIsLocal(buffer))
      {
            MarkLocalBufferDirty(buffer);
            return;
      }

      bufHdr = &BufferDescriptors[buffer - 1];

      Assert(PrivateRefCount[buffer - 1] > 0);
      /* here, either share or exclusive lock is OK */
      Assert(LWLockHeldByMe(bufHdr->content_lock));

      /*
       * This routine might get called many times on the same page, if we are
       * making the first scan after commit of an xact that added/deleted many
       * tuples.  So, be as quick as we can if the buffer is already dirty.  We
       * do this by not acquiring spinlock if it looks like the status bits are
       * already OK.    (Note it is okay if someone else clears BM_JUST_DIRTIED
       * immediately after we look, because the buffer content update is already
       * done and will be reflected in the I/O.)
       */
      if ((bufHdr->flags & (BM_DIRTY | BM_JUST_DIRTIED)) !=
            (BM_DIRTY | BM_JUST_DIRTIED))
      {
            LockBufHdr(bufHdr);
            Assert(bufHdr->refcount > 0);
            if (!(bufHdr->flags & BM_DIRTY) && VacuumCostActive)
                  VacuumCostBalance += VacuumCostPageDirty;
            bufHdr->flags |= (BM_DIRTY | BM_JUST_DIRTIED);
            UnlockBufHdr(bufHdr);
      }
}

/*
 * Release buffer content locks for shared buffers.
 *
 * Used to clean up after errors.
 *
 * Currently, we can expect that lwlock.c's LWLockReleaseAll() took care
 * of releasing buffer content locks per se; the only thing we need to deal
 * with here is clearing any PIN_COUNT request that was in progress.
 */
void
UnlockBuffers(void)
{
      volatile BufferDesc *buf = PinCountWaitBuf;

      if (buf)
      {
            LockBufHdr(buf);

            /*
             * Don't complain if flag bit not set; it could have been reset but we
             * got a cancel/die interrupt before getting the signal.
             */
            if ((buf->flags & BM_PIN_COUNT_WAITER) != 0 &&
                  buf->wait_backend_pid == MyProcPid)
                  buf->flags &= ~BM_PIN_COUNT_WAITER;

            UnlockBufHdr(buf);

            PinCountWaitBuf = NULL;
      }
}

/*
 * Acquire or release the content_lock for the buffer.
 */
void
LockBuffer(Buffer buffer, int mode)
{
      volatile BufferDesc *buf;

      Assert(BufferIsValid(buffer));
      if (BufferIsLocal(buffer))
            return;                             /* local buffers need no lock */

      buf = &(BufferDescriptors[buffer - 1]);

      if (mode == BUFFER_LOCK_UNLOCK)
            LWLockRelease(buf->content_lock);
      else if (mode == BUFFER_LOCK_SHARE)
            LWLockAcquire(buf->content_lock, LW_SHARED);
      else if (mode == BUFFER_LOCK_EXCLUSIVE)
            LWLockAcquire(buf->content_lock, LW_EXCLUSIVE);
      else
            elog(ERROR, "unrecognized buffer lock mode: %d", mode);
}

/*
 * Acquire the content_lock for the buffer, but only if we don't have to wait.
 *
 * This assumes the caller wants BUFFER_LOCK_EXCLUSIVE mode.
 */
bool
ConditionalLockBuffer(Buffer buffer)
{
      volatile BufferDesc *buf;

      Assert(BufferIsValid(buffer));
      if (BufferIsLocal(buffer))
            return true;                  /* act as though we got it */

      buf = &(BufferDescriptors[buffer - 1]);

      return LWLockConditionalAcquire(buf->content_lock, LW_EXCLUSIVE);
}

/*
 * LockBufferForCleanup - lock a buffer in preparation for deleting items
 *
 * Items may be deleted from a disk page only when the caller (a) holds an
 * exclusive lock on the buffer and (b) has observed that no other backend
 * holds a pin on the buffer.  If there is a pin, then the other backend
 * might have a pointer into the buffer (for example, a heapscan reference
 * to an item --- see README for more details).  It's OK if a pin is added
 * after the cleanup starts, however; the newly-arrived backend will be
 * unable to look at the page until we release the exclusive lock.
 *
 * To implement this protocol, a would-be deleter must pin the buffer and
 * then call LockBufferForCleanup().  LockBufferForCleanup() is similar to
 * LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE), except that it loops until
 * it has successfully observed pin count = 1.
 */
void
LockBufferForCleanup(Buffer buffer)
{
      volatile BufferDesc *bufHdr;

      Assert(BufferIsValid(buffer));
      Assert(PinCountWaitBuf == NULL);

      if (BufferIsLocal(buffer))
      {
            /* There should be exactly one pin */
            if (LocalRefCount[-buffer - 1] != 1)
                  elog(ERROR, "incorrect local pin count: %d",
                         LocalRefCount[-buffer - 1]);
            /* Nobody else to wait for */
            return;
      }

      /* There should be exactly one local pin */
      if (PrivateRefCount[buffer - 1] != 1)
            elog(ERROR, "incorrect local pin count: %d",
                   PrivateRefCount[buffer - 1]);

      bufHdr = &BufferDescriptors[buffer - 1];

      for (;;)
      {
            /* Try to acquire lock */
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
            LockBufHdr(bufHdr);
            Assert(bufHdr->refcount > 0);
            if (bufHdr->refcount == 1)
            {
                  /* Successfully acquired exclusive lock with pincount 1 */
                  UnlockBufHdr(bufHdr);
                  return;
            }
            /* Failed, so mark myself as waiting for pincount 1 */
            if (bufHdr->flags & BM_PIN_COUNT_WAITER)
            {
                  UnlockBufHdr(bufHdr);
                  LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
                  elog(ERROR, "multiple backends attempting to wait for pincount 1");
            }
            bufHdr->wait_backend_pid = MyProcPid;
            bufHdr->flags |= BM_PIN_COUNT_WAITER;
            PinCountWaitBuf = bufHdr;
            UnlockBufHdr(bufHdr);
            LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
            /* Wait to be signaled by UnpinBuffer() */
            ProcWaitForSignal();
            PinCountWaitBuf = NULL;
            /* Loop back and try again */
      }
}

/*
 * ConditionalLockBufferForCleanup - as above, but don't wait to get the lock
 *
 * We won't loop, but just check once to see if the pin count is OK.  If
 * not, return FALSE with no lock held.
 */
bool
ConditionalLockBufferForCleanup(Buffer buffer)
{
      volatile BufferDesc *bufHdr;

      Assert(BufferIsValid(buffer));

      if (BufferIsLocal(buffer))
      {
            /* There should be exactly one pin */
            Assert(LocalRefCount[-buffer - 1] > 0);
            if (LocalRefCount[-buffer - 1] != 1)
                  return false;
            /* Nobody else to wait for */
            return true;
      }

      /* There should be exactly one local pin */
      Assert(PrivateRefCount[buffer - 1] > 0);
      if (PrivateRefCount[buffer - 1] != 1)
            return false;

      /* Try to acquire lock */
      if (!ConditionalLockBuffer(buffer))
            return false;

      bufHdr = &BufferDescriptors[buffer - 1];
      LockBufHdr(bufHdr);
      Assert(bufHdr->refcount > 0);
      if (bufHdr->refcount == 1)
      {
            /* Successfully acquired exclusive lock with pincount 1 */
            UnlockBufHdr(bufHdr);
            return true;
      }

      /* Failed, so release the lock */
      UnlockBufHdr(bufHdr);
      LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
      return false;
}


/*
 *    Functions for buffer I/O handling
 *
 *    Note: We assume that nested buffer I/O never occurs.
 *    i.e at most one io_in_progress lock is held per proc.
 *
 *    Also note that these are used only for shared buffers, not local ones.
 */

/*
 * WaitIO -- Block until the IO_IN_PROGRESS flag on 'buf' is cleared.
 */
static void
WaitIO(volatile BufferDesc *buf)
{
      /*
       * Changed to wait until there's no IO - Inoue 01/13/2000
       *
       * Note this is *necessary* because an error abort in the process doing
       * I/O could release the io_in_progress_lock prematurely. See
       * AbortBufferIO.
       */
      for (;;)
      {
            BufFlags    sv_flags;

            /*
             * It may not be necessary to acquire the spinlock to check the flag
             * here, but since this test is essential for correctness, we'd better
             * play it safe.
             */
            LockBufHdr(buf);
            sv_flags = buf->flags;
            UnlockBufHdr(buf);
            if (!(sv_flags & BM_IO_IN_PROGRESS))
                  break;
            LWLockAcquire(buf->io_in_progress_lock, LW_SHARED);
            LWLockRelease(buf->io_in_progress_lock);
      }
}

/*
 * StartBufferIO: begin I/O on this buffer
 *    (Assumptions)
 *    My process is executing no IO
 *    The buffer is Pinned
 *
 * In some scenarios there are race conditions in which multiple backends
 * could attempt the same I/O operation concurrently.  If someone else
 * has already started I/O on this buffer then we will block on the
 * io_in_progress lock until he's done.
 *
 * Input operations are only attempted on buffers that are not BM_VALID,
 * and output operations only on buffers that are BM_VALID and BM_DIRTY,
 * so we can always tell if the work is already done.
 *
 * Returns TRUE if we successfully marked the buffer as I/O busy,
 * FALSE if someone else already did the work.
 */
static bool
StartBufferIO(volatile BufferDesc *buf, bool forInput)
{
      Assert(!InProgressBuf);

      for (;;)
      {
            /*
             * Grab the io_in_progress lock so that other processes can wait for
             * me to finish the I/O.
             */
            LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);

            LockBufHdr(buf);

            if (!(buf->flags & BM_IO_IN_PROGRESS))
                  break;

            /*
             * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
             * lock isn't held is if the process doing the I/O is recovering from
             * an error (see AbortBufferIO).  If that's the case, we must wait for
             * him to get unwedged.
             */
            UnlockBufHdr(buf);
            LWLockRelease(buf->io_in_progress_lock);
            WaitIO(buf);
      }

      /* Once we get here, there is definitely no I/O active on this buffer */

      if (forInput ? (buf->flags & BM_VALID) : !(buf->flags & BM_DIRTY))
      {
            /* someone else already did the I/O */
            UnlockBufHdr(buf);
            LWLockRelease(buf->io_in_progress_lock);
            return false;
      }

      buf->flags |= BM_IO_IN_PROGRESS;

      UnlockBufHdr(buf);

      InProgressBuf = buf;
      IsForInput = forInput;

      return true;
}

/*
 * TerminateBufferIO: release a buffer we were doing I/O on
 *    (Assumptions)
 *    My process is executing IO for the buffer
 *    BM_IO_IN_PROGRESS bit is set for the buffer
 *    We hold the buffer's io_in_progress lock
 *    The buffer is Pinned
 *
 * If clear_dirty is TRUE and BM_JUST_DIRTIED is not set, we clear the
 * buffer's BM_DIRTY flag.  This is appropriate when terminating a
 * successful write.  The check on BM_JUST_DIRTIED is necessary to avoid
 * marking the buffer clean if it was re-dirtied while we were writing.
 *
 * set_flag_bits gets ORed into the buffer's flags.  It must include
 * BM_IO_ERROR in a failure case.  For successful completion it could
 * be 0, or BM_VALID if we just finished reading in the page.
 */
static void
TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
                          int set_flag_bits)
{
      Assert(buf == InProgressBuf);

      LockBufHdr(buf);

      Assert(buf->flags & BM_IO_IN_PROGRESS);
      buf->flags &= ~(BM_IO_IN_PROGRESS | BM_IO_ERROR);
      if (clear_dirty && !(buf->flags & BM_JUST_DIRTIED))
            buf->flags &= ~(BM_DIRTY | BM_CHECKPOINT_NEEDED);
      buf->flags |= set_flag_bits;

      UnlockBufHdr(buf);

      InProgressBuf = NULL;

      LWLockRelease(buf->io_in_progress_lock);
}

/*
 * AbortBufferIO: Clean up any active buffer I/O after an error.
 *
 *    All LWLocks we might have held have been released,
 *    but we haven't yet released buffer pins, so the buffer is still pinned.
 *
 *    If I/O was in progress, we always set BM_IO_ERROR, even though it's
 *    possible the error condition wasn't related to the I/O.
 */
void
AbortBufferIO(void)
{
      volatile BufferDesc *buf = InProgressBuf;

      if (buf)
      {
            /*
             * Since LWLockReleaseAll has already been called, we're not holding
             * the buffer's io_in_progress_lock. We have to re-acquire it so that
             * we can use TerminateBufferIO. Anyone who's executing WaitIO on the
             * buffer will be in a busy spin until we succeed in doing this.
             */
            LWLockAcquire(buf->io_in_progress_lock, LW_EXCLUSIVE);

            LockBufHdr(buf);
            Assert(buf->flags & BM_IO_IN_PROGRESS);
            if (IsForInput)
            {
                  Assert(!(buf->flags & BM_DIRTY));
                  /* We'd better not think buffer is valid yet */
                  Assert(!(buf->flags & BM_VALID));
                  UnlockBufHdr(buf);
            }
            else
            {
                  BufFlags    sv_flags;

                  sv_flags = buf->flags;
                  Assert(sv_flags & BM_DIRTY);
                  UnlockBufHdr(buf);
                  /* Issue notice if this is not the first failure... */
                  if (sv_flags & BM_IO_ERROR)
                  {
                        /* Buffer is pinned, so we can read tag without spinlock */
                        char     *path = relpath(buf->tag.rnode, buf->tag.forkNum);

                        ereport(WARNING,
                                    (errcode(ERRCODE_IO_ERROR),
                                     errmsg("could not write block %u of %s",
                                                buf->tag.blockNum, path),
                                     errdetail("Multiple failures --- write error might be permanent.")));
                        pfree(path);
                  }
            }
            TerminateBufferIO(buf, false, BM_IO_ERROR);
      }
}

/*
 * Error context callback for errors occurring during buffer writes.
 */
static void
buffer_write_error_callback(void *arg)
{
      volatile BufferDesc *bufHdr = (volatile BufferDesc *) arg;

      /* Buffer is pinned, so we can read the tag without locking the spinlock */
      if (bufHdr != NULL)
      {
            char     *path = relpath(bufHdr->tag.rnode, bufHdr->tag.forkNum);

            errcontext("writing block %u of relation %s",
                           bufHdr->tag.blockNum, path);
            pfree(path);
      }
}

Generated by  Doxygen 1.6.0   Back to index